Carbon Cycling, Carbon Storage, and Greenhouse Gas Dynamics in Temperate Southern Ontario Swamps

Abstract

Swamps are treed wetlands and account for the highest areal extent of wetland types in Southern Ontario due to the conversion of other wetlands for development and agriculture. However, little is known about the environmental factors and processes that influence carbon (C) dynamics or the resulting C stocks and fluxes therein. Additionally, historically soils were considered the primary or only source of methane (CH4) efflux from swamps, but recent studies have shown that tree stems contribute a significant amount of the total ecosystem CH4 emissions. Swamps are relatively understudied in Canada, therefore uncertainties remain in the true extent of swamps nationally, that, combined with the lack of data, has limited their inclusion in C accounting, climate modelling, and policy.

A multi-year study of temperate swamps was conducted across southern Ontario to assess the exchange of C, carbon dioxide (CO2) and CH4 between soil, trees, and atmosphere, and estimate current C stocks in soil and biomass. To do so, we began with a synoptic study of 12 swamps over 12 months that looked at the effects of hydrogeomorphic setting and canopy type on measurable variables of water table and soil temperature and the subsequent effects on CH4 and CO2. We found mean growing season soil respiration of CO2 and CH4 across the swamps ranged from 11–49 g m-2 d-1 and -3–44 mg m-2 d-1, respectively; biomass stored 7.92–37.0 kg m-2 of C across the sites, and soil stored 11.6–35.5 kg m-2. Over the study year, the swamps functioned as C sinks and followed similar patterns of relationships to environmental conditions as bogs and fens, with little difference between hydrogeomorphic settings or canopy types.

To assess the contribution of tree stems to CH4 flux, three common genera (Acer, Betula, and Thuja) within 3 of the previous 12 swamps were measured simultaneously with soil fluxes over 21 months. As most CH4 emitted from tree stems in wetlands is produced within the soil and transported up the stem, we also measured soil CH4 fluxes alongside environmental conditions such as water table and soil temperature. We found that stem CH₄ emissions varied substantially among species, sites, and seasons, and were influenced by soil temperature, water-table position, and soil CH₄ availability. However, the importance of these drivers differed among sites and tree species, indicating that methane transport through trees is controlled by interacting environmental and physiological processes rather than a single universal mechanism. The highest mean fluxes were from Acer at 0.23 – 20.82 mg CH4 m-2 of stem yr-1, followed by Betula at 0.09 – 0.89 mg CH4 m-2 yr-1, and Thuja at 0.12 – 0.63 mg CH4 m-2 yr-1.

Finally, these studies were combined to create estimates of the full current C stocks and fluxes for four of the swamps. We found that C storage was dominated by soils at 20 – 86 kg C m-2 of swamp, followed by living trees at 9.8 – 19.9 kg C m-2. Woody debris contained 2.9 – 12.5 kg C m-2, while the contribution from living understory vegetation was minimal at 0.1 kg C m-2 or less. Total C stored within the sites ranged from 45.6 to 108.3 kg C m-2. Trees and understory vegetation took up 619 – 982 g C m-2 of swamp yr-1 but were offset by soil and tree fluxes of 898 – 1213 g C m-2 of swamp yr-1, resulting in three swamps acting as C sources at 25 – 298 and one swamp acting as a C sink at -108 g C m-2 of swamp yr-1. In conclusion, temperate swamps in southern Ontario represent locally important stocks of both biomass and soil C, surpassing both upland forests and other wetland classes. Carbon dynamics in temperate swamps are driven by different controls across timescales, with long-term C storage determined by peat accumulation and climate, while short-term greenhouse gas fluxes are regulated by temperature-moisture interactions that respond to the changing seasons and annual weather. As a result, future warming may increase CH4 emissions, particularly if conditions are able to sustain high water tables that support anaerobic processes. Conversely, increased prolonged dry periods may enhance CO2 losses while limiting CH4 emissions, potentially shifting the balance of GHG outputs, but could be offset by enhanced overstory productivity and litter inputs. Fragmentation, conversion, and drainage result in isolated remnants, and these remnant swamps still account for a large portion of wetland in southern Ontario. Both within and surrounding swamps, anthropogenic disturbances may create drier conditions, in turn increasing soil C oxidation and reducing C stores and peat accumulation ability. Incorporating these data into C accounting and modelling will help inform future policies of wetland conservation and restoration and management of swamps as natural climate solutions.